The invention relates to a web-shaped or tubular packaging film, in particular a sausage casing, comprising a carrier layer of fiber-reinforced cellulose, and a resin coating of at least two layers of vinylidene chloride copolymers, and to a process for the production thereof.
A tubular casing, in particular a sausage casing, which is produced from a film web, has been disclosed by European Patent Application No. 0,054,162. For this purpose, the film web is formed to give a tube and the two edge regions, brought into proximity of each other, are joined by means of a film strip, the film strip being sealed against the gas-impermeable sealable surface layer of the film web. Owing to its oxygen-, water- and water vapor-impermeable layer, this tubular casing is suitable, for example, as a sausage casing for cooked and scalded sausages.
By contrast, the film of fiber-reinforced cellulose according to European Patent Application No. 0,149,071 shows, in addition to a closed barrier layer, also a thermoplastic synthetic resin layer with linear interruptions on the other surface. This known film can also be used for the poduction of tubes with a sealed seam.
These sausage casings with a barrier layer can be shirred by conventional processes. For this purpose, relatively long lengths of tubing are conveyed in the direction of the longitudinal axes and shirred in the inflated state against a counter-force. The length of the shirred casing, which is also called a stick, is usually no more than 1 to 3% of the original tubing length. In use, sausage emulsion is continuously injected by means of sausage-filling machines into the stick, which is closed at one end, the tubes being unshirred again.
It has been known for a long time that barrier layers on the surface of cellulose tubes are susceptible to mechanical stresses and can easily suffer damage. A mechanical stress arises in particular on shirring, above all in the region of the edges of the tube folds, where the tube and also the surface layer are sharply kinked. However, the tube coatng frequently does not break during or immediately after the shirring process, but initially shows satisfactory behavior. It is only on storage of the shirred tube that the coating then tends to break at the most highly stressed points. This causes an uncontrollable and undesired permeability of the coating for water vapor, oxygen and aroma substances, so that the durability and quality of the sausage are impaired. This effect is ascribed to the tendency of polymers containing a high proportion of vinylidene chloride to embrittle after the formation of an initially supple film. This aging process starts when the film is formed and reaches its end point after a few weeks, when the coating has lost it original suppleness and extensibility. To avoid these disadvantages, a quaternary copolymer of vinylidene chloride, acrylonitrile, acrylic acid and methyl acrylate is to be used according to German Pat. No. 2,512,994 as the coating material for shirred tubular casings. These coatings produce permanently flexible coatings on the inside of cellulose tubes.
As has now been found, these polyvinylidene chloride copolymers, which form relatively flexible coatings, are rather less suitable for the production of shirred tubular casings with an external barrier layer. The coating tends to break at the kink points, the cause of this not being immediately obvious. The risk of the coating breaking at the kink points also exists when tubes with an outer barrier layer are stored, being wound up in the flattened state.